Precipitation behavior in the early stage of aging in an Al–Li°Cu–Mg–Zr–Ag (Weldalite 049) alloy

The precipitation behavior of various phases during the aging process of an Ag–Li°Cu–Mg–Zr–Ag (Weldalite 049) alloy was investigated by high-resolution electron microscopy and in situ hot-stage microscopy. Two kinds of domains with L12-type ordered structures, which are considered to be δ′ and β′ phases, are observed with different domain sizes in the alloy quenched from 530 °C. In the early stage of aging at 190 °C, the δ′ phase is precipitated as surrounding the β' phase, and the δ′ domains appear with in-phase and antiphase relationships to the β′ lattices. In situ observations at 190 °C clearly show that the T1 phase precipitates predominantly on dislocations at subgrain boundaries and then is homogeneously formed in the matrix with increasing aging time. The nucleation of the S′ phase is associated with clustering of Cu and Mg in the matrix, and the S0 domains are grown with {210} habit planes.